In water-based paper laminating adhesive applications, it is often necessary for the adhesive to have both (1) a high degree of wet tack and (2) a sufficient open tack time (tack range) to bond the substrate quickly and successfully under a light to moderate pressure. However, it has proven to be difficult to develop an adhesive system having both important wet tack characteristics. Several methods have been utilized in the past to achieve optimum wet tack performance properties. For instance, water-based laminating adhesives have been prepared in the presence of boron-modified polyvinyl alcohols to enhance the wet tack properties. However, these adhesives are sensitive to the pH of the system, and tend to form useless gels at relatively high pH. In addition, the wet tack performance of the adhesive could be pH dependent.
Another method to enhance the wet tack performance is to compound solvents or plasticizers into the base adhesives. This method is often used by the end users of the packaging industry. The use of solvents, however, could create environmental concerns and safety issues. Also, the performance of plasticized adhesives could deteriorate because of plasticizer migration. Moreover, formulating the adhesives with solvent or plasticizers will add raw material and operating costs to the converters.
Another important property required by the industry is a relatively fast setting speed, the adhesive's ability to set a bond rapidly and to maintain this bond after a short application of pressure.
As the environmental regulations become more stringent and the current trend of the packaging industry moves toward high-speed applications, it is urgent to develop water-based adhesives having both high wet tack and sufficient tack range properties in addition to fast speed of set to respond the growing needs of the industry.
U.S. Pat. No. 3,827,996 discloses aqueous dispersions of vinyl ester polymers prepared in the presence of a stabilizing amount of polyvinyl alcohol having an average residual vinyl acetate content of 5 to 7 mole %. A single polyvinyl alcohol having this vinyl acetate content may be used or a combination of polyvinyl alcohols having average vinyl acetate contents of up to about 16 mole % may be blended to produce an average to 5 to 7 mole %. The resulting vinyl ester polymer dispersions are said to possess stability, both during and after preparation, water resistance of the adhesive after it has been deposited from the dispersion and a satisfactory viscosity index.
U.S. Pat. No. 4,521,561 discloses a vinyl acetate/ethylene copolymer emulsion exhibiting both partially- and fully-hydrolyzed polyvinyl alcohol compatibility which comprises an aqueous collodial dispersion of a copolymer prepared by the emulsion polymerization of vinyl acetate and ethylene in the presence of a polyvinyl alcohol stabilizing system having an 8 to 10 mole % residual vinyl acetate content, especially comprising a mixture of a polyvinyl alcohol having a 3 to 6 mole % vinyl acetate content and a polyvinyl alcohol having a 10 to 14 mole % vinyl acetate content in the ratio that yields an average residual vinyl acetate content of 8 to 10 mole %. When blended with a partially- or fully-hydrolyzed polyvinyl alcohol to provide an adhesive composition, such compositions demonstrate an enhanced plasticizer thickening.
U.S. Pat. No. 4,605,589 discloses a vinyl acetate/ethylene copolymer binder emulsion prepared by the copolymerization of vinyl acetate and ethylene in the presence of a polyvinyl alcohol which is 50 to 99 mole % hydrolyzed and has a 100 to 375 degree of polymerization (DPn) such that it demonstrates a viscosity of about 2.4 to 4 as a 4% aqueous solution at 20.degree. C. Nonwoven fabrics bonded with such copolymers possess surprisingly improved softness while maintaining toughness.
U.S. Pat. No. 4,043,961 discloses adhesive compositions consisting essentially of an aqueous emulsion of vinyl acetate/ethylene copolymer prepared in the presence of protective colloid comprising fully hydrolyzed vinyl alcohol copolymer containing methyl methacrylate. The examples show the use of such vinyl alcohol copolymer in combination with a medium and/or low viscosity partially hydrolyzed polyvinyl alcohol and a nonionic surfactant.